The invention relates to a meter and a sensor concept, and also to a method for measuring properties of a fluid flow. The meter comprises a sensor, an electronic unit, and a software package, in particular for the continuous measurement of the composition of a fluid flowing through a duct or channel. In this document in particular the use as a downhole meter for the measurement of the water fraction of the fluid being produced in an oil well is described. However, the invention may also be used for measuring other properties and values.
The sensor is based on using both the microwave resonance principle for the measurement of oil-continuous fluids (water drops and gas bubbles in oil, i.e. the oil is the continuous phase), and the measurement of conductivity for water-continuous fluids (oil drops and gas bubbles in water, i.e. the water is the continuous phase). The meter is intended for installation at a production zone inside an oil well.
The meter can also measure the fluid that is being produced from a specific zone. The measurement result can be used for controlling a valve that controls the production rate from the zone. Such equipment is especially useful in so-called smart wells, in which several zones produce into the same well. The in-flow from one zone is being mixed with the main flow (that has been produced from other zones and flows in a tube system) at the valve controlling that zone. The composition ought to be measured inside the well (down-hole) between the perforations in the casing or liner and the valve, while the fluid is flowing in the annulus, before it is being mixed with the main flow. Knowing the composition of the fluid being produced is important for the long-term optimization of the recovery from a zone.
There are a number of different meters on the market for the measurement of the water contents of oil. Some meters are based on the use of radioactive radiation, some are capacitive, and some are based on the use of microwaves. The radioactive sensors are problematic in many environments because of the health risks with such radiation, and the safety measures required. In downhole applications this would encounter a serious problem, particulary during the installation phase. In addition the accuracy is a problem because the radiation is mainly sensitive to differences in density, and the difference in density of oil and water is small or even zero. The capacitive sensors measure the permittivity (ref. chapt. 2 in Nyfors E., and P. Vainikainen, Industrial Microwave Sensors, Norwood, Mass.: ArtechHouse, 1989) of the fluid at frequencies that are much lower than those used by microwave sensors. They are therefore very sensitive to all kinds of contamination, as a thin layer of e.g. scale or wax has a large influence on the impedance of such sensors. They also require a relatively complicated mechanical structure including a dielectric protecting cover on the inside of the sensor, so that the electrodes do not come in direct contact with the fluid that is measured. Microwave sensors do not have these problems. A microwave sensor measures the permittivity of a fluid. Because the permittivity of water is much higher than that of most other substances, oil included, the permittivity of a fluid containing water is very sensitive to the water content.
Currently there are no sensors of any kind available on the market for the measurement of the water content of fluids neither in the annular space between two pipes, nor downhole in an oilwell.
The conditions that a meter for measuring operations downhole in the annulus of an oilwell has to face are difficult. These are the high pressure (typically up to 1000 Bar), the high temperature (typically up to 180xc2x0 C.), the very limited space, the low electrical power available (because of the typically several km long supply cable), and the high required reliability because of no possibilities to service the meter once it has been installed with the completion of the oilwell. These conditions require a specially designed microwave sensor for the annulus. The sensor must also be designed so that a measurement principle can be used that requires an absolut minimum amount of electronics at the downhole location of the sensor because of the limited space, the limited power, and the requirement on reliability in combination with the high temperature. The measurement method should also be such that a minimum amount of information needs to be transferred to the surface, because of the low data handling capacity of a several kilometers long combined power/signal cable.
The resonant frequency of a microwave resonator sensor can be measured with basically two different methods (ref. Vainikainen, P., xe2x80x9cMeasurement electronics of industrial microwave resonator sensorsxe2x80x9d, Thesis for the degree of Doctor of Technology, Helsinki University of Technology, Radio Laboratory, Report S 194, 1991).
The first involves measuring the frequency response of the resonator by performing a frequency sweep, with e.g. a VCO (voltage controlled oscillator). The resonant frequency is then usually derived by performing a curve fit. If this method would be used in a downhole application, either the whole frequency response, typically involving hundreds of measurement points, would have to be transferred to the surface for one measurement of the water content, or there would have to be a data processing capability in the downhole electronics. Transferring the frequency response to the surface is slow, and processing the data downhole would make the electronic unit much more complex and unreliable.
The second is the so-called FSA method (feedback self-oscillating amplifier), which is based on locking an oscillator to the resonant frequency of the sensor. The FSA method is fast and simple. The frequency only needs to be counted downhole and transferred to the surface as a single number for each measurement of the water content. The FSA method requires a pure frequency response in the sense that there should be no other resonance peaks near the one used, so that it can be assured that the electronics always locks to the right resonance peak.
This invented sensor is a microwave cavity resonator of a new design, in particular adapted for downhole use, e.g. in an oil/gas well. The advantages are that it is suitable for permanent installation in an annulus, the frequency response that makes it suitable for measurement with the FSA method, a simple structure, and probes that can also be used for measuring the conductivity of the fluid for determining the water content, when the fluid is water-continuous. With these features a minimum amount of electronics is required downhole, which allows for obtaining a good reliability also at the high temperatures encountered in the downhole environment. A minimum amount of data transmission is needed, which enables for fast measurements. All software for performing the necessary calculations of the water content can be located at the surface.
These advantages can be obtained by using the techniques, features and methods according to the claims stated below.